Battery apparatuses for use as power supplies in smart grids or vehicles such as electric cars, each apparatus comprising battery units connected together, has been proposed in recent years.
Any battery apparatus comprises many battery cells connected in series and in parallel. More precisely, as many battery cells (for example, 10 cells) as required to achieve safety even if they are short-circuited are connected in series, forming one row. Several cell rows are connected in parallel, constituting one assembled battery. Then, some assembled batteries are connected in series, providing a battery unit. Further, some battery units are connected in parallel, thereby providing a battery apparatus.
The main-circuit output (i.e., output at the anode and cathode terminals) of the battery apparatus are connected to a power conditioner (PCS), and a charge/discharge current flows in the battery apparatus. A large-scale battery system may comprise such battery apparatuses connected in parallel.
Lithium ion batteries may constitute a battery apparatus. In this case, the batteries must be monitored in terms of voltage and temperature, and the difference between the batteries in terms of remaining power must be reduced, in order to use the battery apparatus in safety for a long time. Further, each battery module must be charged or discharged, or must not be charged or discharged, in accordance with the estimated amount of remaining power and the result of self-examination of the module. To this end, assembled battery monitoring devoices (i.e., cell management units, CMU) are used for respective assembled batteries, and are connected by communication lines.